1. Field of the Invention
The present invention relates generally to a space-time block coding (STBC) apparatus and method for a transmitter in a wireless communication system, and in particular, to an STBC apparatus and method for maximizing coding advantage (or coding gain) and achieving full diversity and full rate in a mobile communication system using two transmit (Tx) antennas.
2. Description of the Related Art
The basic issue in wireless communications is how efficiently and reliably to transmit data on channels. Along with the demand for a high-speed communication system capable of processing and transmitting video and wireless data in addition to the traditional voice service, future-generation multimedia mobile communication systems, now under active study, increase system efficiency using an appropriate channel coding scheme.
Generally, in the wireless channel environment of a mobile communication system, unlike that of a wired channel environment, a transmission signal inevitably experiences loss due to several factors such as multipath interference, shadowing, wave attenuation, time-variant noise, and fading.
The information loss causes a severe distortion in the actual transmission signal, degrading the whole system performance. In order to reduce the information loss, many error control techniques are adopted, depending on the characteristics of channels, to increase system reliability. The basic error control technique is to use an error correction code.
Multipath fading is relieved by diversity techniques in the wireless communication system. The diversity techniques are classified as time diversity, frequency diversity, and antenna diversity.
The antenna diversity technique uses multiple antennas. This diversity scheme is further branched into receive (Rx) antenna diversity using a plurality f Rx antennas, Tx antenna diversity using a plurality of Tx antennas, and multiple-input multiple-output (MIMO) using a plurality of Tx antennas and a plurality of Rx antennas.
The MIMO is a special case of space-time block coding (STC) that extends coding in the time domain to the space domain by transmission of a signal encoded in a predetermined coding method through a plurality of Tx antennas, with the aim to achieve a lower error rate.
V. Tarokh, et al. proposed STBC as one of methods of efficiently achieving antenna diversity (see “Space-Time Block Coding from Orthogonal Designs”, IEEE Trans. On Info., Theory, Vol. 45, pp. 1456-1467, July 1999). The Tarokh STBC scheme is an extension of the Tx antenna diversity scheme of S. M. Alamouti (see, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Selected Area in Communications, Vol. 16, pp.1451-1458, October 1988), for two or more Tx antennas. In addition, the use of spatial multiplexing (SM) aiming to increase data rate leads to capacity that is linearly proportional to the number of Tx antennas.
FIG. 1 is a block diagram of a transmitter in a mobile communication system using the conventional STBC scheme proposed by Tarokh. The transmitter is comprised of a modulator 100, a serial-to-parallel (S/P) converter 102, an STBC coder 104, and four Tx antennas 106, 108, 110 and 112.
Referring to FIG. 1, the modulator 100 modulates input information data (or coded data) in a predetermined modulation scheme. The modulation scheme can be one of binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), pulse amplitude modulation (PAM), and phase shift keying (PSK).
The S/P converter 102 parallelizes serial modulation symbols, s1, s2, s3, s4 received from the modulator 100. The STBC coder 104 creates eight symbol combinations by STBC-encoding the four modulation symbols, s1, s2, s3, s4 and sequentially transmits them through the four Tx antennas 106 to 112. A coding matrix used to generate the eight symbol combinations is expressed by Equation 1:
      G    4    =      [                                        s            1                                                s            2                                                s            3                                                s            4                                                            -                          s              2                                                            s            1                                                -                          s              4                                                            s            3                                                            -                          s              3                                                            s            4                                                s            1                                                -                          s              2                                                                        -                          s              4                                                            -                          s              3                                                            s            2                                                s            1                                                            s            1            *                                                s            2            *                                                s            3            *                                                s            4            *                                                            -                          s              2              *                                                            s            1            *                                                -                          s              4              *                                                            s            3            *                                                            -                          s              3              *                                                            s            4            *                                                s            1            *                                                -                          s              2              *                                                                        -                          s              4              *                                                            -                          s              3              *                                                            s            2            *                                                s            1            *                                ]  where G4 denotes the coding matrix for symbols transmitted through the four Tx antennas 106 to 112 and s1, s2, s3, s4 denote the input four symbols to be transmitted. The number of the columns of the coding matrix is equal to that of the Tx antennas and the number of the rows corresponds to the time required to transmit the four symbols. Thus, the four symbols are transmitted through the four Tx antennas for eight time intervals.
Specifically, for a first time interval, s1 is transmitted through the first Tx antenna 106, s2 through the second Tx antenna 108, s3 through the third Tx antenna 110, and s4 through the fourth Tx antenna 112. In this manner, −s4*, −s3*, s2*, −s1* are transmitted through the first to fourth Tx antennas 106 to 112, respectively for an eighth time interval. That is, the STBC coder 104 sequentially provides the symbols of an ith column in the coding matrix to an ith Tx antenna.
As described above, the STBC coder 104 generates the eight symbol sequences using the input four symbols and their conjugates and negatives and transmits them through the four Tx antennas 106 to 112 for eight time intervals. Since the symbol sequences output to the respective Tx antennas, that is, the columns of the coding matrix are mutually orthogonal, as high a diversity gain as a diversity order is achieved.
FIG. 2 is a block diagram of a receiver in the mobile communication system using the conventional STBC scheme. The receiver is the counterpart of the transmitter illustrated in FIG. 1.
The receiver is comprised of a plurality of Rx antennas 200 to 202, a channel estimator 204, a signal combiner 206, a detector 208, a parallel-to-serial (P/S) converter 210, and a demodulator 212. The first to Pth Rx antennas 200 to 202 provide signals received from the four Tx antennas of the transmitter illustrated in FIG. 1 to the channel estimator 204 and the signal combiner 206. The channel estimator 204 estimates channel coefficients representing channel gains from the Tx antennas 106 to 112 to the Rx antennas 200 to 202 using the signals received from the first to Pth Rx antennas 200 to 202. The signal combiner 206 combines the signals received from the first to Pth Rx antennas 200 to 202 with the channel coefficients estimated by the channel estimator 209 in a predetermined method. The detector 208 generates hypothesis symbols by multiplying the combined symbols by the channel coefficients, calculates decision statistics for all possible transmitted symbols from the transmitter using the hypothesis symbols, and detects the actual transmitted symbols through threshold detection. The P/S converter 210 serializes the parallel symbols received from the detector 208. The demodulator 212 demodulates the serial symbol sequence in a predetermined demodulation method, thereby recovering the original information bits.
As stated earlier, the Alamouti STBC technique offers the benefit of achieving as high a diversity order as the number of Tx antennas, namely a full diversity order, without sacrificing the data rate by transmitting complex symbols through two Tx antennas only.
The Tarokh STBC scheme, which is an extension of the Alamouti STBC scheme, achieves a full diversity order using a space-time block code in the form of a matrix with orthogonal columns, as described with reference to FIGS. 1 and 2. However, because four complex symbols are transmitted for eight time intervals, the Tarokh STBC scheme brings a decrease by half in the data rate. In addition, since it takes eight time intervals to completely transmit one block with four complex symbols, reception performance is degraded due to channel changes within the block over a fast fading channel. In other words, the transmission of complex symbols through four or more Tx antennas requires 2N time intervals for N symbols, causing a longer latency and a decrease in the data rate.
FIG. 3 is a block diagram of a conventional SM transmitter for 5 increasing data rate. Referring to FIG. 3, an S/P converter 303 vertically distributes signals modulated in a modulator 301 to Tx antennas 305, for transmission. The use of the thus-configured transmitter requires a number of (Rx) antennas equal to or greater than the number of the Tx antennas 305. While four Tx antennas are used in the illustrated case, the SM technique is applicable to any number of Tx antennas. In particular, Diagonal Bell Labs Layered Space Time (D-BLAST) and Vertical-BLAST (V-BLSAT) processing techniques are available according to S/P conversion and receiver configuration. These space-time processing techniques are applications of successive interference cancellation (SIC) to receivers. They are suboptimal solutions.
As is clear from the above description of the conventional techniques, an STBC apparatus and method have yet to be developed which achieve full diversity and full rate simultaneously for two Tx antennas. In a mobile communication system with two Tx antennas, the full diversity is [2×number of Rx antennas] (2 is the number of Tx antennas) and the full rate is 2. The use of two Tx antennas increases the data rate and the increase is double that available in a single-input single-output (SISO) system.
Accordingly, a need exists for developing a full-diversity, full-rate STBC apparatus and method in a mobile communication system using two Tx antennas.
Another need exists for developing an STBC apparatus and method that maximize coding gain in a mobile communication system using two Tx antennas.